Fractures that occur in proximity to a joint can be difficult to treat. Although plates, screws and pins on the surface of the bone can provide fracture stability, often the close proximity of tendons to the surface of the bone can result in soft tissue irritation and even tendon rupture that can compromise the outcome. Intramedullary fixation of fractures, with or without cross-locking screws, is well known to reduce the problem of soft tissue irritation by placing the bulk of the implant within the bone itself.
Furthermore, intramedullary fixation can provide fracture stability because of either a tight fit of the rod within the bone or fixation from locking screws that cross through the bone and rod. Traditional intramedullary rods, however, are not well suited for fixation of a fracture in proximity to the end of the bone. For example, in the case of fractures of the distal radius, the distal end of the radius is extremely wide with soft cancellous bone within the intraosseous space and only thin weak cortical bone that surrounds the tubular structure; the strongest bone at the distal end of the radius is the thick subchondral bone that extends behind the articular surface and is under the tip of the radial styloid. In the case of fractures of the distal radius, insertion of the implant is not possible through the tubular proximal fragment because of its deep location and the narrow, cylindrical nature of the morphology that makes it impossible to direct an intramedullary implant down the center of the bone. Because of this, intramedullary fixation of distal radius fractures has always inserted the device directly through the soft radial surface of the radial styloid in order to direct the implant within the intramedullary canal. This necessarily results in creation of a large additional hole that is at least the diameter of the implant in the small distal fragment, which can easily result in creation of additional fragmentation, collapse of the fragment and resultant loss of fixation.
In addition, since the implant is inserted through this large defect in the distal fragment, it is not possible for the end of the implant to be used to provide axial support to the fragment; instead, the implant is totally dependent on the resistance of the thin cortical bone to translational movement and the purchase of transverse locking screws in the soft, often osteoporotic, metaphyseal bone. As a result, loss of radial length can easily occur, resulting in protrusion of the nail from the insertion site as well as deformity and loss of function. Finally, because the end of the bone is often covered with articular cartilage which is damaged if a nail is inserted through it, standard intramedullary implants are unable to provide support to the end of the bone as they must be placed more proximal to this area to prevent damage to the joint.
Standard intramedullary rods use cross locking screws to prevent the small distal fragment from losing length. Examples of standard intramedullary rods are shown in U.S. patent application Ser. No. 10/377,255 to Warburton and entitled Intramedullary Interlocking Fixation Device for the Distal Radius (U.S. Publication No. 2004/0010255) and U.S. patent application Ser. No. 09/975,514 to Putnam and entitled Intramedullary Rod for Wrist Fixation (U.S. Publication No. 2003/0073999). Because these screws are placed across the nail into the metaphyseal bone of the distal fragment, they are loaded at their tip by the compressive loads that occur across the wrist. This places a significant torque on the screw, which can lead to increased implant loads and can result in breakage, cutout through the bone, or loosening of the screw. In turn these can result in loss of length, deformity, and impaired function of the wrist.
Since the distal radius is made of relatively soft cancellous bone, there is little resistance to side-to-side translational displacements by a standard intramedually nail, particularly since the nail is placed through a hole made in the bone and courses to lie entirely within the metaphyseal cavity. This results in poor support of the fragment by the nail itself, requiring the majority of resistance to displacement to be taken up by the distal crossing screws.
In copending U.S. patent application Ser. No. 10/675,864 to Medoff and entitled Intramedullary Implant for Fracture Fixation (U.S. Publication No. 2005/0070902), an approach was described that provides axial support of the radial styloid by the tip of an implant that is placed intramedullary into the distal fragment. Since the implant lies on the extraosseous surface proximally, the implant enters the fracture site and can be placed with a single longitudinal insertion into the distal fragment. However, this design requires a more extensive dissection for placement of the extramedullary portion of the implant in addition to resulting in an implant that is still fixed on the surface of the bone over one part, with the possibility of further soft tissue irritation. In addition, since the surface portion of the implant must be thin to avoid prominence and soft tissue irritation, this creates a stress riser at the junction of the extramedullary and intramedullary portions of the implant that can result in breakage.
Current intramedullary implants are inserted into a tubular bone from one end and driven to the opposite end. In some applications, the implant is inserted at the proximal end of the bone and driven in an ante grade direction into the distal end of the long bone. In other applications, the implant is inserted at the distal end of a long bone and driven in a retrograde direction into the proximal end. Because the direction of insertion is always uni-directional, current intramedullary designs do not permit fixation both above and below the site of insertion of the implant. In addition, since existing intramedullary implants are designed for insertion in a single direction only (either ante grade or retrograde), these implants are always connected to a driver at one end.